In recent years, technical development on radar apparatuses installed in vehicles (hereinafter referred to as on-vehicle radar apparatuses) is actively underway. As one of the examples, a radar apparatus using a spread spectrum method (hereinafter referred to as a spread spectrum radar apparatus) and the like has been suggested (for example, refer to Patent Reference 1).
On-vehicle radar apparatuses are used to detect a preceding vehicle, a rear obstacle, and the like, for the purpose of safety improvement such as collision avoidance and enhancement of the driving convenience represented by reverse driving support. When the on-vehicle radar apparatuses are used for such purposes, it is necessary to suppress an effect of an undesired radio wave generated from other radar apparatuses of the same type installed in vehicles other than the own vehicle, such as an effect of interference by an electromagnetic wave.
In contrast, since a transmission radio wave is modulated in a spread spectrum radar apparatus using a PN code for spectrum spreading, a signal caused by a radio wave which is modulated in a different code and a signal outputted from a radar apparatus using another modulation method which does not use code are suppressed within a receiver. Furthermore, since the transmission radio wave is frequency-spread using the PN code, it is possible to reduce the electric power on a frequency unit basis and thus to reduce the effect on other wireless systems. It is also possible to freely set a relationship between distance resolution and a maximum detectable range by adjusting a chip rate and a code cycle of the PN code. The peak power never becomes large since continuous transmission of a radio wave is possible. Note that an undesired radio wave mixed into a transmission radio wave is spread over a wide band in a frequency domain even when a despreading process is performed, and unnecessary noise and an interference signal are suppressed using a filter for a narrow band.
Furthermore, the spread spectrum radar apparatuses are classified into a heterodyne method type and a homodyne method type depending on the configuration of the receiver.
The heterodyne method is a method of receiving a signal that converts a frequency into an intermediate frequency by mixing (multiplying) a reception signal and a signal having a frequency different from a frequency of the corresponding transmission signal by a predetermined frequency, and that performs signal processing, such as amplification and wave detection.
The homodyne method is a method of receiving a signal that directly obtains a baseband signal by mixing (multiplying) a reception signal and a signal having the same frequency as that of the transmission signal.
However, according to the aforementioned conventional technology, a frequency stabilizer using a phase-locked loop is necessary in each reception side and transmission side of the spread spectrum radar apparatus using a heterodyne method, and thus, there is a problem that it is difficult to lower the price of the apparatus. This is because it is necessary to control, in high precision, an oscillation frequency of local oscillators at the reception side and transmission side and to sufficiently stabilize, according to a pass band of a filter, an intermediate frequency which is a frequency difference between the reception side and transmission side. However, there is a characteristic that a problem to be described hereinafter which is unique to the homodyne method does not occur in a spread spectrum radar apparatus using the heterodyne method.
On the other hand, the spread spectrum radar apparatus using the homodyne method has problems, such as characteristic variations in semiconductor devices, direct current offsets caused by the variation in the ambient temperature, and thus, it is necessary to install a direct current amplifier. Since the detection range of the radar apparatus is set wide, it becomes a substantial obstacle when the dynamic range of the receiver is increased. By using a local oscillator which is common in the reception and transmission sides, it is possible to ease a degree of frequency stability required in an oscillator. This is possible because an output signal obtained by wave detection from the receiver includes a direct current component.
In order to solve this problem, the method of embedding data code in the PN code has been conceived (for example, refer to Patent Reference 2). The method has a characteristic in that the problem of the direct current offset does not occur in a receiver by using a local oscillator which is common in the reception and transmission sides.    Patent Reference 1: Japanese Unexamined Patent Application Publication No. 7-12930    Patent Reference 2: Japanese Unexamined Patent Application Publication No. 10-54874